Weather permitting, I love to throw open every window in the house. Indoor air can be stagnant and even more polluted than what’s outside, so every now and then, it’s nice to feel the cleansing effects of a fresh breeze, especially at night. Until the sounds of traffic, lawn mowers, or the neighbor’s barking dog start to seep in as well.
Apparently, I’m not the only one who thinks it’s silly that outdoor air and a quiet house are mutually exclusive. A team of South Korean engineers are using acoustic metamaterials to create a window that reduce environmental noise while still allowing air to pass through into the house.
The study, published recently in arXiv, proposes a window consisting of “a three-dimensional array of strong diﬀraction-type resonators with many holes centered at each individual resonator.” According to the authors, the unique design of the window, which is based on two wave theories of diffraction and acoustic metamaterials, is capable of filtering out unwanted noises while still allowing the free pass-through of air.
“Sound waves of 80dB in the frequency range of 400 − 5,000Hz were applied to the windows,” write the researchers. “It was observed that the sound level was reduced by about 30 − 35dB in the above frequency range with the 20mm window and by about 20 − 35dB in the frequency range of 700 − 2,200Hz with the 50mm window.”
The results, observed by Professor Sang-Hoon Kim of the Mokpo National Maritime University and Professor Seong-Hyun Lee of the Korea Institute of Machinery and Materials in South Korea, are about five dB better than that of a standard vinyl double-pane window.
So how do the diffraction resonators pass air but not sound? Gizmag explains:
One part of the answer is that when sound passes through the air holes, the sound waves are strongly diffracted into the entire volume of the diffraction resonator, so that very little of the sound can pass directly through the air holes. The other part of the answer is that the diffraction resonators cause the air to have negative compressibility over a fairly wide frequency band.
The technology is actually quite simple, and is only slightly different than windows already in use, so it may not be long until they’re available. Read the full study here [PDF].